An important objective of the invention is to provide a relatively simple and inexpensive system which enables available and inexpensive thermistors to be used to provide highly accurate temperature readings in one or more relatively narrow temperature ranges, and which is also capable of providing reasonably accurate temperature readings over extended temperature ranges.
A constructed embodiment of the invention has found particular use in medical laboratories for providing highly accurate temperature readings in the vicinities of 25.degree. C., 30.degree. C. and 37.degree. C. In such an embodiment the center values are set at an expanded scale of 24.degree.-26.degree. C., 29.degree.-31.degree. C. and 36.degree.-38.degree. C., and a single scaling value is adjusted to make the readings accurate over their entire ranges. Similarly, the center value may be set on a wide range of, for example, 20.degree.-40.degree. C., and by reading one other point, the system may be calibrated accurately over the entire scale.
As stated above, the output signals from the bridge are digitized to create addresses for temperature values stored in memory. This serves to convert voltages that are non-linear, because of the characteristics of the thermistor and of the bridge, into accurate temperature readings. This linearizing technique is similar in some respects to the linearizing methods described in U.S. Pat. Nos. 3,824,585 and 4,169,380.
A feature of the system of the invention is its ability to provide accurate temperature readings over an extended range with high sensitivity. High sensitivity is obtained by the use of a thermistor sensor. The accuracy at the center of a selected temperature range is obtained by nulling the bridge circuit controlled by the thermistor at the center of the temperature range. The accuracy over the full scale is obtained by using the PROM table look-up technique, which, as mentioned, is similar to that described in the patents referred to above.
The PROM table is calculated by using the equation representing the basic characteristic of the thermistor, and the bridge, namely: ##EQU1## Where: E.sub.out =amplifier output voltage;
E.sub.bridge =voltage applied to the bridge; PA1 R.sub.o =thermistor resistance when E.sub.out is zero; PA1 R.sub.x =thermistor resistance at temperature T, and is given by equation (2); PA1 R.sub.Feedback =resistance of feedback resistor connected between the amplifier output and the amplifier negative input. ##EQU2## Where: T=temperature of thermistor; PA1 T.sub.o =temperature of thermistor when thermistor resistance is R.sub.o ; PA1 a, b & c=characteristic values of thermistor material.
The PROM will then linearize any of a family of thermistors independently of the nominal resistance of the thermistor. That is, different thermistors fabricated from the same material but with different nominal resistance values may be used in the bridge circuit of the system of the invention without changing the linearizing table. For each thermistor, one adjustment is made to adjust the system gain, and a second adjustment is made to null the bridge circuit at the center of each temperature range for that particular thermistor.
Thermistors are available which are made of the same basic material, and such thermistors are usually inexpensive, especially if their nominal resistance values at the reference temperature (usually 25.degree. C.) are not too critical.
As mentioned above, the PROM is actually used to correct for non-linearities both of the thermistor and also of the bridge circuit. The system to be described is equipped with range switches which enable several ranges of interest to be linearized using multiple look-up tables. The system described has three scales which require only a single system gain adjustment. However, a second system gain adjustment is used for an extended range (20-40 degree C.) to prevent the bridge amplifier from saturating.